The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
For the production of hydrocarbon wells, boreholes are drilled into subterranean formations. Following standard procedures, a fluid is circulated during drilling from the surface through the interior of the drill string and the annulus between drill string and formation. The drilling fluid, also referred to as “drilling mud”, is used to accomplish a number of interrelated functions, such as suspending and transporting solid particles, such as drill cuttings, to the surface for screening out and disposal. The fluid may also transport a clay or other substance capable of adhering to and coating the uncased borehole surface, both (a) to exclude unwanted fluids which may be encountered, such as brines, thereby preventing them from mixing with and degrading the rheological profile of the drilling mud, as well as (b) to prevent the loss of downhole pressure from fluid loss should the borehole traverse an interval of porous formation material. The fluid may keep suspended an additive weighting agent (to increase specific gravity of the mud), typically barites (a barium sulfate ore, ground to a fine particular size), so that the entire column of drilling fluid is not interrupted upon encountering pressurized pockets of combustible gas, which otherwise would tend to reduce downhole pressure, as well as creating a “blowout” in which the fluid and even the drill stem are violently ejected from the well, with resulting catastrophic damages. The fluid may also constantly lubricate the drill bit so as to promote drilling efficiency and retard bit wear.
Drilling fluids may also be used to provide sufficient hydrostatic pressure in the well to prevent the influx and efflux of formation fluids and drilling fluids, respectively. When the pore pressure (the pressure in the formation pore space provided by the formation fluids) exceeds the pressure in the open wellbore, the formation fluids tend to flow from the formation into the open wellbore. Therefore, the pressure in the open wellbore is typically maintained at a higher pressure than the pore pressure. While it is highly advantageous to maintain the wellbore pressures above the pore pressure, on the other hand, if the pressure exerted by the drilling fluids exceeds the fracture resistance of the formation, a formation fracture and thus induced mud losses may occur, in a circumstance known as lost circulation. Further, with a formation fracture, when the drilling fluid in the annulus flows into the fracture, the loss of drilling fluid may cause the hydrostatic pressure in the wellbore to decrease, which may in turn also allow formation fluids to enter the wellbore. As a result, the formation fracture pressure typically defines an upper limit for allowable wellbore pressure in an open wellbore while the pore pressure defines a lower limit. Therefore, a major constraint on well design and selection of drilling fluids is the balance between varying pore pressures and formation fracture pressures or fracture gradients through the depth of the well.
Lost circulation is a recurring drilling problem, characterized by loss of drilling mud into subterranean formations. However, in addition to drilling fluids, lost circulation may remain an issue for other fluids such as including completion, drill-in, production fluid, etc.
Lost circulation may result from induced pressure during drilling, as described above. A particularly challenging situation arises in depleted reservoirs, in which the drop in pore pressure weakens hydrocarbon-bearing rocks, but neighboring or inter-bedded low permeability rocks, such as shales, maintain their pore pressure. This can make the drilling of certain depleted zones very difficult because the mud weight required to support the shale exceeds the fracture resistance of nearby zones composed of weakly consolidated sands and silts. Another unintentional method by which lost circulation can result is through the inability to remove low and high gravity solids from fluids. Without being able to remove such solids, the fluid density can increase, thereby increasing the hole pressure, and if such hole pressure exceeds the formation fracture pressure, fractures and fluid loss can result. Further, fluid loss can occur naturally in earthen formations that are naturally fractured, highly permeable, porous, cavernous, or vugular. These earth formations can include shale, sands, gravel, shell beds, reef deposits, limestone, dolomite, and chalk, among others.
The industry typically distinguishes between three classes of drilling fluids: oil-based, water-based and so-called synthetic muds. Oil-based and synthetics muds are recognized for their superior qualities for most of the drilling operations. The drilling fluid compositions may further include weighting agents, surfactants, proppants, viscosifiers, and polymers. The loss of drilling fluid into fractures is a major concern in the well construction process, and one class of additives often used includes fluid loss agents useful to prevent the drilling fluid from entering into porous or fractured formations. While fluid loss agents are designed to form a competent filter cake to cover porous formations, there is another class of materials referred to as lost circulation materials. Lost circulation materials are designed to prevent or limit fluid losses into fractures or other openings in the formation. Lost circulation materials are typically particulates that invade the fracture and at some point along the fracture, form a blocking bridge across the fracture. When the fracture is bridged a filter cake may develop due to the pressure difference between the wellbore fluid and the fluid beyond the fracture block. The pressure beyond the filtercake or bridge block is reduced and thus may prevent the full borehole pressure from affecting the tip of the fracture. However any further increase in the wellbore fluid pressure may result in further opening of the fracture leading to renewed losses.
Accordingly, there exists a continuing need for developments of new lost circulation materials, and treatments that may be used during a lost circulation event so that circulation may be more readily resumed, such need met at least in part by embodiments disclosed herein.